Power driven finish sanders of the vibrating or orbital type such as the Model 330 "Speed-Block".TM. quarter sheet sander or the Model 505 "Heavy Duty" half sheet sander sold by Porter-Cable Corp., Jackson, Tenn., originally were sold with felt back up pads to back and support coated abrasive sheets driven by the sanders. Typically such pads had rectangular spaced parallel major surfaces, one of which was driven in a small reciprocating or orbital pattern by a drive mechanism in the sander, and the other of which supported the sheet of abrasive coated material. The sheet of coated abrasive material was held along the surface on which it was supported by clamps at the ends of the pad that engaged end portions of the sheet of abrasive coated material that extend around opposite ends of the pad.
I started a project to substitute for Porter-Cable's felt back up pad a back up pad to which abrasive sheets coated with pressure sensitive adhesive (e.g., "Stikit".TM. coated abrasive sheets available from Minnesota Mining and Manufacturing Company) could be releasably adhered . It was found that the physical properties of the back up pad had a significant effect on the ability of the sanders to perform effectively. When the back up pad included a layer of a highly resilient or bouncy elastic material such as the neoprene foam rubbers having shore A compression readings of 3 and 1 respectively which are respectively commercially designated R411N and G231-N and are available from Rubatex Corporation, Bedford, Va., or the closed cell neoprene foam or the open cell polyurethane foams having shore A compression readings of 25 and 8 respectively which are respectively commercially designated SN-430 and HD-150 and are available from Illbruck, U.S.A., Minneapolis, Minn., such resilient or bouncy elastic material efficiently transferred driving forces from one surface attached to the drive mechanism of the sander to another surface along which the coated abrasive sheet was adhered in a direction parallel to those surfaces, but had a strong tendency to cause the tool to chatter and bounce on the workpiece being sanded. Such chatter and bouncing resulted in low cutting efficiency, gouging, and irregular scratch patterns on the workpiece. When the back up pad included a layer of a polymeric material that was relatively soft and had low resiliency or bounce such as the closed cell neoprene rubber foam having a shore A compression reading of 1-2 commercially designated R5010A available from Rubatex Corporation or the closed cell neoprene rubber foam, having a shore A compression reading of 6, the closed cell polyvinyl chloride having a shore A compression reading of 1, or the open cell polyurethane/ether having a shore A compression reading of 1 respectively commercially designated as SN-410; SV-210; and E-290, and available from Illbruck, U.S.A., Minneapolis, Minn., the sanding pad would not cause the tool to chatter and bounce on the workpiece, however, the driving force would not be efficiently transferred from the surface attached to the drive mechanism of the sander to the surface on which the coated abrasive sheet was adhered so that little sanding was done by the coated abrasive. The selection of conventional foam material for use as the pad required a compromise that was not particularly satisfactory in achieving either the optimum transfer of driving forces between those surfaces or in minimizing the tendency of the tool to chatter and bounce on the workpiece, which compromise at best produced results that were not much better than results produced by the use of the original felt pad with the abrasive coated paper clamped along its surface as described above.